Method for surface mount solder joints

Abstract

A method and apparatus for the forming of a solder deposit on an SMD pad on a printed circuit or hybrid board. This formed solder deposit is in a defined three-dimensional well having the proper profile and a defined solder gap. The solder before forming can be solid solder or a solder paste. By the placement of a mesh on the surface of the circuit board with the solder in place on the pads, applying a slight positive pressure on a rigid or elastic surface on the other side of the mesh, and subjecting this system to a temperature just low enough to reflow the solder by means of a heat transfer fluid, then cooling the board to solidify the solder, a product results having the above properties. The duration of the application of heat and pressure is made so brief, that the laminate structure of the board and the coating thereon remain substantially unaffected. A circuit board with the solder so formed thereon.

What is claimed is: 
     
       1. A process for the forming of solder deposits on surface mount device pads comprising the steps of: a) providing a printed circuit board having solder deposited on the pads;   b) covering said board with a mesh material;   c) placing said board with the mesh thereon between surfaces capable of uniform compression;   d) pressing together said surfaces;   e) placing said thus prepared circuit board at any orientation in a heat transfer fluid medium maintained at an elevated temperature whereby the solder melts;   f) removing said circuit board from the medium;   g) cooling said circuit board to ambient temperature while compressed; and   h) removing said circuit board from between the compressive surfaces and the mesh.   
     
     
       2. The process of claim 1 wherein the printed circuit board is a hybrid circuit board. 
     
     
       3. The process of claim 1 wherein the solder deposited is selected from the group consisting of solid solder, solder paste, solder cream and solder powder. 
     
     
       4. The process of claim 1 wherein the steps of removing said circuit board from the medium and cooling further includes the steps of: a) containing the thus-melted solder within the approximate periphery of the pads;   b) moving the thus-melted solder upwards and downwards within the mesh; and   c) imprinting the mesh on the uppermost portion of the solder deposit.   
     
     
       5. The process of claim 1 wherein the mesh material includes counts of 60-300, (292-53 microns Standard Sieve Designation) and thicknesses, wire diameters and weaves consistent with this mesh count. 
     
     
       6. The process of claim 1 wherein the surfaces capable of uniform compression are selected from the group consisting of metal, plastic, glass and rubber. 
     
     
       7. The process of claim 1 further comprising a sheet of elastic material placed between the mesh material and the surfaces capable of uniform compression. 
     
     
       8. The process of claim 1 wherein the pressing is done at a pressure sufficient to form the solder. 
     
     
       9. The process of claim 1 wherein the heat transfer fluid is selected from the group consisting of natural oils and synthetic oils. 
     
     
       10. The process of claim 1 wherein the step of removing said circuit board from between the compressive surfaces and the mesh includes the step of releasing the surface and mesh from the formed solder. 
     
     
       11. The process of claim 1 wherein the heat transfer fluid includes the further property of protecting the printed circuit board including its components from degradation by heat. 
     
     
       12. The process of claim 1 wherein the elevated temperature is maintained between about 370-420 degrees Fahrenheit, sufficient to soften the solder. 
     
     
       13. The process of claim 12 wherein the thus prepared circuit board is maintained at the elevated temperature for about 2-5 minutes. 
     
     
       14. The process of claim 1 wherein the cooling is done in ambient air. 
     
     
       15. The process of claim 1 wherein the cooling is done by placing the heated, compressed circuit board in heat transfer media of progressively lower temperatures. 
     
     
       16. The process of claim 1 wherein the mesh material is selected from the group consisting of stainless steels, glass cloths, chemically milled screens, chemically milled plates and laser-machined plates. 
     
     
       17. A method for forming solder deposits on a printed circuit board having surface mount device pads with the solder deposits thereon, comprising the steps of: a) covering said printed circuit board with a mesh material;   b) placing said mesh material covered printed circuit board between surfaces capable of uniform compression;   c) pressing together said surfaces;   d) heating said thus-prepared printed circuit board uniformly at an elevated temperature such that the solder deposits melt;   e) cooling; and   f) removing said cooled printed circuit board from between said mesh material and the compression surfaces constructed and arranged such that the solder deposits are contained within said pads.   
     
     
       18. The process of claim 1 wherein one of the compression surfaces is the mesh material itself stretched on a weighted chase constructed and arranged such that registration is maintained while forming the softened solder.